Preparation of printed images by electrophotographic, or "xerographic", processes involves coating a selected substrate, or xerographic plate (typically comprised of metal, glass or plastic), with a photoconductive insulating material such as selenium, and then providing an electrostatic charge on the photoconductive surface, e.g., by ionization from a corona discharge. A light image is then focused onto the charged surface, which discharges or lowers the potential of the irradiated areas, while leaving the remainder of the surface charged. The electrostatic image so formed is then made visible by application of a suitable developing composition, which may be in either dry or liquid form.
Conventional liquid developer compositions comprise a dispersion of pigment particles in an insulating carrier liquid. Application of such a composition to the substrate carrying the electrostatic image results in migration of charged pigment particles to the substrate surface and deposition thereon in conformance with the electrostatic image. The developed image is then transferred to another substrate such as paper. (In some cases, it is desirable to eliminate the intermediate step of image transfer, i.e., so that the developed image is directly produced upon the final surface; see, e.g., U.S. Pat. No. 3,052,539 to Greig.)
Liquid developers for use in multicolor image development are relatively recent, and are comprised of colorant embedded in a thermoplastic resin core. These "toner" particles are then dispersed in an insulating carrier medium as above. Like compositions used in black-and-white electrophotography, these developer compositions additionally contain charge control agents to control the charge acquired by the toner particles in the insulating liquid.
When a color image is to be produced electrophotographically, the above-described charging, exposure, and development steps are carried out separately in succession for each of the constituent colors of the image using a correspondingly colored toner. In some color printing processes, each of the color images is transferred from the electrophotographic member to a print substrate after development and prior to formation of the next color image. This process, however, requires extremely accurate registration of the successive color images on the substrate to which they are transferred in order to obtain a high-quality composite image.
Another color printing process, and the process currently in use commercially, is a four-color liquid electrophotographic process known as "consecutive color toning" or "consecutive multicolor image development". This process involves: (1) charging a photoconductive (pc) surface; (2) impressing a first latent image on the surface by exposure through a colored transparency; (3) developing the image by contacting the pc with a liquid developer composition of a first color, typically yellow; and (4) discharging the pc surface. The steps are then repeated in sequence, typically using magenta, cyan, and black developer compositions, i.e., the cyclic process is repeated until the colored image is complete.
A significant problem which has been encountered in consecutive color toning is "image" or "character" staining, that is to say, where a second process color overtones the first image in regions where portions of the first image should have been discharged but were not. See, for additional explanation of the problem, R. M. Schaffert, Electrophotoqraphy (London: Focal Press, 1975), at pp. 184-186.
Many schemes have been advanced to overcome this difficulty. In U.S. Pat. No. 4,701,387 to Alexandrovich et al., for example, the problem of residual toner is discussed. The inventors propose a solution wherein the developed surface is rinsed with a polar liquid after each development step. It is suggested that application of a polar rinse liquid neutralizes and solvates residual counterions deriving from charge control agents and stabilizers present in the liquid developer.
While the Alexandrovich et al. method may be effective in reducing the staining problem, such a multiple washing procedure is time-consuming and unwieldy (it is recommended in the '387 patent that "after each development step and before the next developer is applied, the developed image is rinsed . . . . After rinsing, the rinse liquid is removed from the photoconductive element by drying, wiping or other method . . . "; see col 2, lines 62-67).
U.S. Pat. No. 2,986,521 to Wielicki proposes the use of photoconductive toner particles to permit dissipation of charge applied to a toner layer during exposure of a second or subsequent color image to avoid charge retention in those areas. Such developers, however, may also be sufficiently conductive in the dark to dissipate the charge where it is intended to be retained during a subsequent imaging process, thereby preventing the subsequent image from being developed in those areas. U.S. Pat. No. 3,687,661 to Sato et al. seeks to overcome the problem resulting from retained charge by applying a reverse-polarity charge which neutralizes any charge retained in previously developed regions of the electrophotographic member. Such additional steps, however, not only prolong the processing time required to produce a composite color image, but also add to the complexity of the electrophotographic apparatus.
Other problems frequently encountered in electrophotographic color processes include: background staining, i.e., the appearance of toner in uncharged, non-image areas (a problem which is ubiquitous in zinc oxide and other positive toner systems); poor image resolution (i.e., poor edge acuity); poor image density resulting from insufficient deposition of toner particles in intended image regions; and colorant exposure, in which colorant contained within the resinous toner particles is exposed to the developer solution (as well as to the substrate) and thus affects the chemistry of the particular developer composition.
The invention herein now provides compositions and processes which address and overcome each of the aforementioned problems. First with respect to image staining in multicolor image development, the present toner and developer compositions substantially eliminate the cause of the problem and avoid the time-consuming, multi-step procedures of the prior art. The presently disclosed compositions and processes also enable preparation of a final electrophotographic print of unexpectedly high quality, with respect to both image density and edge acuity. The problems of colorant exposure and background staining are also virtually eliminated as will be described in detail below.